If you need to quickly explain an idea and you have a dry erase marker but no whiteboard, one unnamed reddit user suggests hanging a piece of saran wrap for a quick and disposable whiteboard.

It's as simple as taping a piece of saran wrap to a wall or door. It's by no means a permanent whiteboard, but if you have idea busting out of you and don't have access to a mirror to write on, it works in a pinch.